Author Archive for Andy Murkin

With the help of a correspondent to my blog I was lucky enough to get hold of an early 80’s ‘Stylophone’ from the Soviet Union.

Entitled the ‘Gamma’, I’m told it was made in the city of Chernivtsi, a part of the Soviet Union now in western Ukraine.

Larger than a Dübreq Stylophone, it came in a neat plastic box measuring about 25x20x5cm.

Inside, the Gamma Stylophone itself has a 20-note keyboard at the front, a stylus – more complicated than a Dübreq Stylophone stylus – on a twin lead, a volume control on the left-hand side, and a speaker in the top left-hand corner. A coloured label indicates the notes of the scale represented by each section of the keyboard.

There are also 3 strange slots above the keyboard, which are slightly wider than the keyboard and just deep enough to be accessed by the stylus. This close up of the keyboard shows the middle of two of these slots:

The stylus, as mentioned above, is more complicated than the Dübreq Stylophone stylus in that it includes a combined press and slide switch. It turned out that the press switch had to be pushed for the stylus to work; the slide switch turned the vibrato on or off.

Helpfully, my correspondent had cleaned the instrument before sending it, so there wasn’t a lot for me to do! I opened the case – just 4 slot-headed screws underneath – and examined the insides. The circuit board was attached to 4 mounts on the base; the speaker had 4 mounts on the front.

Turning the circuit board over, I could see the components and the layout. Everything seemed neat and well made, with a good solid loudspeaker.

The components on the circuit board weren’t quite the same as their Western equivalents, but quite recognisable, nonetheless:

I removed a little more of the disintegrating foam – and replaced a speaker wire, which I had inadvertently detached – and then turned to the power cables. The battery fittings had been removed, but I could see from the booklet which came with the instrument, that these had been designed for a pair of Soviet-style 4.5v batteries: my correspondent explained to me that these were similar to a group of three 1.5v batteries – not unlike the kind of thing we used to have inside cordless phones – but which were, in any case, now uncommon.

I just added a PP3 battery clip, similar to the type one would find in an old-style Dübreq Stylophone, which worked fine. I hadn’t intended to ‘circuit-bend’ this device, but I pondered on adding an on/off switch, as there isn’t one in the original design.

I attached the battery and tried it out. The push switch needed a bit of attention – a few squirts of contact cleaner helped – but all the notes sounded perfectly, and the vibrato turned on and off. I wasn’t able to check that the notes were in tune, and there’s no fine tuning control, which you would find on a Dübreq Stylophone, but I’ll look into that later.

The booklet that came with the Gamma Stylophone was delightful – the paper and printing quality weren’t high, but the illustrations were beautiful and the colourful instructions on how to play the many songs were attractively set out.

In the first post in this series, I tested some pickups using various inductors, which picked up electrical noises from my laptop.

One of my long-term projects is field recording, which I began by using a Marantz PMD-660 solid-state recorder. This track is based on the first series of recordings I made, at a nearby lock – although much of the track uses these recordings altered by Karlheinz Essl’s application fLOW, which I have discussed before:

I improved later recordings with some good quality microphones which I bought from a guy connected with the Wildlife Recording Society who makes them himself.

I decided after a while to expand the range of field recordings I could make, by creating stereo hydrophone and contact mics. I’ve described these in use here, and written about constructing them earlier in the blog.

Finally, I decided to add a fourth type of recording, using inductors of the type I had experimented with earlier.

I used the same transistor-based preamp I had experimented with – the one I originally made for the electret elements.

As with the hydrophone and contact mics, I put the preamp in a small plastic case with appropriate in and out connectors, and a 3.5mm socket and velcro patch for the external 9v battery.

I wanted the inductor pickups to be a little more robust for outdoor use, so I used two of the ‘telephone pickup’ coils I described in the first post, without removing them from their plastic cases.

I attached them to a shielded stereo phono lead and for a holder, I chose a folding plastic ruler. The idea of this was that the two coils could be moved closer together or further apart to maximise the stereo effect of any electrical sounds they picked up.

With the help of some epoxy adhesive and superglue I attached the telephone coils to the plastic ruler. As they were still inside their plastic containers, they would be sufficiently robust and weatherproof for outdoor use.

I went out recording by the river the other day and came across a lamp post and a large electrical box, connected with some flood gates. The inductor recorder picked up these sounds:

This device may have a limited application in comparison to standard microphones or the other recording devices I’ve made recently – the hydrophone and contact mics referred to above – but it has a place in my collection and I’m sure in time I’ll find plenty of interesting sources of electrical noise to record on my travels.

Finally this summer I was able to try out my hydrophones and contact mics outdoors, in making some field recordings in and beside a nearby river.

I began with the hydrophones, which I described in the last post in this series. My experience with the 50mm piezo discs was that there was too much pickup from the cables and the wooden float structure in the bath, so I took the mics made from the 35mm discs.

These worked fine in absolutely still water, but whenever there was the slightest wind or wave – which was virtually all the time – there was too much noise from the wind or water hitting the cables and the wooden float, so the wooden structure had to be discarded.

I was rather restricted then in only being able to record where I could reach and dangle the mics in the water, but the recordings were much better, and even worked well with the discs lying on the river bed. I believe the sound at the end of this recording is a water snail munching on a reed stem: I saw the snail, which was quite large, an inch or two in length, and dangled the microphone close to it; this is what I heard:

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I then turned to recording with the contact mics. The basic format of these, and the preamp I used, are described in this post, Pt 2 of this series.

Experimenting beforehand with the sturdy clips and clamps I had bought for outdoor use – pictured in Pt 3 of this series, here – I found that pressure on the ‘crystal’ side of the disc, where the leads are attached, had a tendency to cause distortion, so I decided for the outdoor version of the contact mics I would use the same ‘sandwich’-style construction as for the hydrophones, with 2 discs mounted back-to-back (or front-to-front – that is, with the crystal side inside), but kept apart by a ring of silicone sealant around the edge:

The difference in this case, though, was that the leads were attached to only one of the discs – I removed the leads from the other one as I had not experienced the same noise problems as I had with the hydrophones, and it wouldn’t therefore be necessary to have the double leads from each mic and the 4 channel balanced preamp which the hydrophone required.

The preamp was not the same one I had used for the experiments in the posts referred to above – although it was similar. This time I used a standard inverting op amp amplifier. The resistor on the input was 1M and the resistor between the input and output was 10M, providing amplification of up to 10 times, but the output volume was controlled by a dual-gang 10k log potentiometer. I was intending to use my usual TL072 dual op amp, which is pretty low-noise and would have worked fine, but in the end used an NE5532 as I had just bought some of these and they are known for being especially low-noise. They are also a pin-for-pin replacement for the TL072.

I enclosed the circuit in a small plastic box, very similar in appearance to the hydrophone preamp. This design was easy to hold in the hand and manipulate the volume control while monitoring with headphones.

As previously mentioned, it was quite a windy day when I went out, so I attached the contact mics to tree branches to test them out – one mic would be near the end of the branch, the other one closer to the trunk. One disc would be held tight to the branch (the side to which the leads were attached), and the clamps pressed firmly onto the disc without leads. This side, of course, needn’t have been a piezo disc at all, but I used them because they were exactly the same size as the discs with the leads, and these particular discs were very inexpensive, so I didn’t feel it was too much of a waste of resources. If you didn’t want to use up piezo discs like this, you could use any fairly rigid substance – plastic or glass, for example – as long as it was roughly the size of the disc and wouldn’t deform when clipped or clamped.

As I hoped, there was no distortion caused by pressure on the crystal layer, and the recordings came out well, capturing the movement of the trees in the wind.

In my next post in this series, I’ll try out some improved methods of recording closer to where I want to in the water.

While making the piezo contact microphones described earlier in this series, I decided to adapt a couple of them for the specific purpose of recording sound underwater in rivers and ponds.

In this case, I wanted them to be compatible with my Marantz PD660 recorder, which records in stereo via two XLR external microphone sockets.

For my first attempt, I began by using 3m twin shielded cables, which had two 6.35mm mono jacks on one end – what was on the other end didn’t matter, as these connectors were cut off and the ends attached to piezo elements. As with the others, the shields were attached to the brass outsides, the cores to the centres, and they were given two coats of Plasti-Dip.

In this way I was able to record in stereo, but my attempts were bedevilled by extraneous noise. I also realised that underwater recording required much greater amplification than I was getting from the simple preamp I had used for the simple percussion instruments I had been making.

Looking around the internet to see what others had done, I came across the idea on this site of using two piezos back-to-back for each channel. Actually, this is probably more accurately described as front-to-front, as it requires the two piezos to be sandwiched together with flexible silicone sealant.

Having read somewhere else that the air gap behind a piezo affects what it picks up, I decided it would better if there was some air between the two piezos, rather than completely filling the space between them with the sealant. Hence, I just put a ring of sealant round the edges before sticking the discs together.

The main thing is to keep the centres of the two discs from touching.

To connect the discs to the recorder, I used a 3m twin XLR cable. Each channel used two discs: the black leads from the discs were connected together, and the two red leads were connected to the left and right connectors of the XLR lead.

In fact, I made two sets, to see if there would be a difference between 35mm discs and 50mm discs; the 35mm discs are shown above. The 50mm discs came with no wires attached, so I had to solder the wires on myself – a bit of a tricky job, to make sure they attached firmly, but without damaging the delicate central area of the disc with the crystals in it. When I had done the soldering and checked the strength of the joint, I superglued the wires to the outer edge of the discs to make sure there would be no stress on the soldered joints which might cause the wires to become detached while in use.

In the case of the 50mm discs, where there was more space, I stuck a small lead weight on the edge of one disc in each pair before putting them together. This, I hoped, would make them more stable in the water. Once again, I used the same principle of putting the sealant round the edges of the discs only, and then putting them together like a sandwich.

I connected the wires to a twin XLR cable as before, and gave the discs two coats of Plasti-Dip.

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In yet another place I had read that underwater signals might require up to 100 times amplification, so I put together a TL072-based buffer amp somewhat similar to the preamp I had used for the piezo-based percussion instruments, but with two major differences:

Firstly, it was designed to amplify the input signal by a factor of 100, with a volume control to reduce the output if necessary; secondly, it was in the form of a ‘differential’ amplifier, so that the signals on the left and right leads of each channel were amplified and added together, producing a simple two-channel stereo output from the four inputs.

Each channel looked like this:

After checking the prototype (pictured above) was working satisfactorily, I improved the connections with 2 core shielded cable and put the circuit into a small box. I changed the output from the 3.5mm socket shown to a 5 pin XLR socket because this matched the professionally made apparatus I use for field recording with conventional microphones. There was no room for a battery in the box, so I used the 3.5mm socket for a power socket.

For absolute minimal noise, the closer the preamp is to the piezos the better, and I have seen designs where the circuit was contained in a waterproof housing in the water. However, mine is designed so the preamp box is well away from the water, and the output volume control can be used while it’s in operation, as well as the input volume control on the recorder.

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The first problem when it came to testing the hydrophones was how to ensure that they floated in the correct place; and the second was how to maintain a consistent stereo image.

Clearly, the hydrophones would require some sort of framework to which they could be attached, and from which their depth underwater could be adjusted. The framework would need to be buoyant and, as I discovered, potentially very wide. The speed of sound in air is about 750mph or 350 metres per second, and a distance between the microphones of about 9-12 inches or 25-30cm is usually enough to get a satisfactory stereo image. However, underwater, the speed of sound is more like 1,500 metres a second, getting on for four and a half times as fast!

This meant that the hydrophones would need to be spaced up to four and a half times the distance apart to get the same stereo effect as two conventional microphones recording in the open air.

I thought about different methods of creating a frame 3 – 4 feet or 1 – 1.5 meters long, and easily foldable for transportation, and came up with the idea of using a folding wooden ruler. I found one which was a metre long, about the minimum necessary length, which folded on brass hinges into 5 sections of 20cm each. This would easily fit into a bag and could be carried to the recording site before being opened out and placed in the water. It cost about £2 (although postage was £3!).

The first thing I did with the ruler was cut a number of slots in it:

The purpose of these slots was so that the hydrophone leads could be threaded through them, stay in position, and allow the piezo elements to stay at the right depth in the water beneath the framework.

I tested that the slots were suitable for this purpose:

and then – since I had the tin in the workroom – gave the framework a coating of Plasti-Dip. I’m sure it would have been fine without it, but the Plasti-Dip will hopefully give the wood a little extra protection, make it easier to dry and increase its life.

Later, because of using a wider construction with two piezos sandwiched together, I cut narrower channels to connect the slots to the edge of the framework, so the wires could fit in and the piezos wouldn’t need to pass through the slots.

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The yellow balls in the above picture need an explanation! Although the wood would be quite buoyant, the framework would need to be supported in the water. I looked to see what anglers might use, and came across these small ‘bubble floats’:

These are hollow plastic and have a small bung in them so they can be partly filled with water. This gives them a little weight so they can be cast into an ideal position in the water, but will still float. I thought these would ensure the wooden framework would remain on the surface and the hydrophones could be positioned at an appropriate depth beneath.

In the end, I used them without any water in them; empty, they were able to support the framework slightly under the surface. As can be seen in the picture, in a restricted space such as a bath or small pond, the end sections of the framework can be folded in.

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Having fitted the floats, it was time to test the device, for which purpose I filled my bath, floated the framework with the piezos attached, and ran the tap. The sound was pretty good, and the noise minimal:

I’ll post again when I’ve had a chance to try it out in the field!

[Edit: the wooden float wasn’t very successful in the field. See next post in the series]

They were not guaranteed to work, but experiments showed that what mostly seemed to be wrong with them was that the batteries had run down – in fact, when I examined the box that I kept them in recently, quite a lot of them were leaking and spoiling the plastic cases.

However, there was no reason to think that the circuit boards inside were damaged. In fact, a few years ago, shortly after I bought them, I’d successfully used a couple of boards – without really looking into their function fully – in a circuit-bent instrument which I called the StyloSound.

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I thought now would be a good time to sort them out properly and put more of them to use, so I began sorting through them, setting aside the old batteries for recycling, discarding damaged cases and keeping useful parts. After a while the floor of my workroom looked like this:

Clockwise from the top can be seen: complete units in good condition (batteries apart); small 8ohm speakers; small 6.5mm electret elements; intact circuit boards; discharged AG13 coin batteries; key rings.

Some of these items would be of use in this project. I have already described in the earlier post referred to above, using the electrets in constructing percussion instruments with plastic bottles; I was now hoping to use the circuit boards in all the instruments to give them a recording and looping function.

In the top picture below, the main chip can be seen – or, rather, can’t be seen, as it’s hidden under the black blob in the centre.

Above the chip are the two tracks which were beneath the record/play button; and in the top left the switch which selects between these two functions. I didn’t intend to use the selector switch, but two buttons, one side connected to the ‘rec’ side of that switch in one case, and the ‘play’ side in the other, and the other side connected to the large track, should do the job.

In both pictures the LED is visible on the opposite side to the ‘Play’/’Rec’ switch. This is connected to light up when the ‘Record’ button is pressed.

I also made sure to make notes of connections to the board before removing external components.

As it was under a protective blob, it was impossible to say which chip it was which was employed in this circuit. However, it was quite possibly one of the ISD1800 series – in particular the ISD1820, about which there is quite a lot of information on the internet (for example here and here), and using which there are quite a number of modules available.

These are very reasonably priced at £1 or less – although not nearly as cheap as my voice recorder boards, as long as I could get them to work in the way I wanted. Note in the picture above that, although the ISD1820 is usually shown as a 16-pin DIL chip, the modules on the right use a smaller, concealed version, so I thought at first this might be the one on the voice recorder boards I was proposing to use.

One of the features of the ISD1820 is that it has two methods of playback. In one case it will play a recording as long as the ‘Play’ button is pressed – in the same way as it will make a recording as long as the ‘Record’ button is pressed; but in the other, as soon as the ‘Play’ button is pressed, the recording will play through to the end, even if the button is immediately released.

Pressing ‘Play’ once on these devices I had was enough to cause the recording to play through to the end, which is what I had hoped.

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However, this wasn’t everything I was looking for. In the StyloSound, the ‘Play’ button needs to be pressed every time playback of the recorded sound is required. Although this is suitable for the StyloSound, it isn’t proper looping, where the sound will be repeated indefinitely. The ISD1820 has a method of triggering repeats, and I was hoping I would find a way of doing this in a similar way with the boards I had.

In the ISD1820 looping is achieved by linking the pin that lights up the LED to the ‘Play’ button. I was hoping this would be the case with these boards.

First I connected the power, audio in and out sockets, and ‘Play’ and ‘Record’ buttons – one side to the points each side of the switch, the other side to the large track above the blob – to check that the board was functioning correctly. The recording quality wasn’t that great, but it recorded and played back without problems.

I then connected the ‘Play’ button to the LED connection – but no luck, it didn’t cause the recording to repeat: so I got out my multi-meter and connected one side to 0v pressing the ‘Play’ button and testing with the other side to try and find a point on the board which would be at a high voltage while the recording was playing and then went low as soon as it had finished – this high-to-low change being the thing which would trigger the recording to start playback again. I found a spot, and connected this to the ‘Play’ button, made a new recording and pressed ‘Play’. This time the recording played and repeated continuously.

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I went round each of the 5 mono instruments and connected a voice recorder PCB. The first example, the one for the ‘Snare’ instrument, looked like this:

1 is the ‘Record’/’Play’ switch. To conserve space, I decided to use a special switch which I had a small bag of. This was a 3-way SPDT toggle, one way for ‘Record’, the other way for ‘Play’, with a centre off position.

2 is a 10 ohm resistor, connecting the output to +v. I remembered that I’d had problems with the output of the board when I used it in the Stylosound, and this was the same – when I connected the board to the input of the TL072 mixer, there was no output. I reasoned, in the light of further experience, that this could have been that the circuit required a load to compensate for the speaker which had been removed. The speaker was 8 ohms, so I connected a 10 ohm resistor in its place. Sure enough, the output came through loud and clear . . . even though the output of the board wasn’t even connected to the mixer! I have no idea why this happened, but it did. It worked, so I didn’t look into it any further.

3 is a 10k volume control I added to the output of the mixer. Like most of the pots in this project, it was salvaged from the PCBs of some supposedly non-working mixers I had bought as a job lot from eBay (previously described here). As with the voice recorders, some of them worked fine, or had minor faults, but a couple of them were good only for parts.

4 is the new connection point for the looping function.

5 are the connections for ‘Record’ and ‘Play’.

After these pictures were taken I also removed the LED from the board, and attached it with longer wires, so that it could be mounted beside the new ‘Record’/’Play’ switch.

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Finally it was time to finish these instruments off.

I began with the piezo instruments, installing the switches, potentiometers and LED, tucking the electronics away underneath, and attaching feet to the base:

The electret instruments proved to be slightly more complicated. Firstly. I had to add a TL072 mixer, as described in the previous post in this series, but also I needed to use the other half of the TL072 – which is a dual op amp – to double the output from the electret preamp.

The layout of this circuit was exactly the same as the mixer, except that it had a single input, from the output of the electret preamp, via a 100k resistor, and the resistance between the input and output was 200k, amplifying the input by 2. To save space, I didn’t use pieces of strip board, I just added the appropriate resistors to 8-pin ic sockets for each of the instruments into which the TL072’s were plugged.

A 1M resistor was needed between the output of the PT2399 and the input of the mixer to balance the level with the output of the electret amplifier, which, as with the piezo circuits, used a resistance of 100k.

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I took the opportunity at this point to make a suitable 4.5v power supply for the percussion units, using a spare wooden base. Some time ago I had bought a hundred 3.5mm sockets for 10-15p each, and I still had quite a few left, so I used a dozen of these for the outputs – more than enough for the modules I’d made so far, with a few spare for future additions. I found a socket which matched an old mains adapter I had been given, and added a small, low-cost voltage regulator module, an LED scavenged from one of the defunct mini-mixer PCBs, and 4 plastic feet:

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Here are some recordings I made with some of the finished instruments and the new power supply:

The echo seemed quite good, although by no means noise-free; the looper was less effective. The straightforward amplified sound was excellent in each case; the degree to which the extra circuits were practical or useful varied from one instrument to another.

For a further use of piezo elements as a pickup – in this case a hydrophone for underwater recording – see Pt. 7 of this series.

In the previous post in the piezo series I had prepared 4 percussion instruments, all based on making sounds to be picked up by piezo discs. In the picture below, the one on the left has 4 lengths of piano wire soldered to a large (50mm diameter) piezo; the second one has a snare from a snare drum attached to a large, shallow tin; the third one has an adjustable rubber-lined clip designed to hold a Latin American-style rainstick; and the right-hand one is a circle of sandpaper with a piezo disc firmly superglued to the back.

In the electret series I had made 2 new instruments in which sounds would be picked up by electret elements, and had identified 2 existing instruments, a xylophone and glockenspiel, which needed amplifying in a similar way. Each of the instruments had its own appropriate preamp, either a piezo type or electret type, as described earlier in the series.

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Next, one of the things I felt percussion instruments would benefit from was a reverb/delay circuit, and I had been looking around for a long time for something suitable. In particular I was looking for a circuit that would be inexpensive, simple to put together, and easily repeatable in different units I might construct or circuit-bend.

Once I came across the PT2399 chip, I knew I’d found the answer.

According to the datasheet, the PT2399 is ‘an echo audio processor IC utilizing CMOS Technology which is equipped with ADC and DAC, high sampling frequency and an internal memory of 44K. Digital processing is used to generate the delay time, it also features an internal VCO circuit in the system clock, thereby making the frequency easily adjustable. PT2399 boast very low distortion (THD<0.5%) and very low noise (No<-90dBV), thus producing high quality audio output. The pin assignments and application circuit are optimized for easy PCB layout and cost saving advantage.’

I simplified the circuit even more, and adjusted some of the capacitor values to lengthen the delay time and keep the noise to a minimum, and ended up with this first version:

I haven’t labelled it, but the volume pot is 10k. Ideally, this should be a log pot, although lin pots are usually cheaper and easier to come by these days. The component values are all very standardised, as I bought these values in bulk – using either resistors or capacitors in series or parallel, you can get close to other typical values. These values worked fine for me in this context, however. The value of 50k (lin) was chosen for the ‘Delay Time’ control as it gave a very wide range of delay times.

In this configuration, the repeat function is fully on, but when the ‘Delay’ control is turned fully anti-clockwise, there are no repeats.

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One of the slight problems with the PT2399 is its very strict voltage requirements: below about 4.5v it’s unlikely to work; above 6v and it will probably be damaged. I tested it with three 1.5v batteries, which was my original intended power supply for the percussion instruments, and it worked fine.

I first wanted to build a stand-alone circuit, however, and for this I used the usual 9v PP3 battery and a voltage regulator to reduce 9v to 5v, which is the PT2399’s preferred supply.

The voltage regulators were small DC-DC modules I bought for about 45p each, so didn’t increase the cost of the device too much. These modules will accept an input voltage of up to 28v, and the output voltage can be adjusted from about 1v to 20v.

This view of the finished circuit shows how I put it together: there is no circuit board, but most of the components are soldered to the pins of the i.c. socket holding the PT2399.

Made in this way, the circuit would take up very little space in, for example, the restricted space of a circuit-bent keyboard or toy. The 10uF DC-blocking capacitors and the volume control might also not be needed in some applications. It might be somewhat lo-fi, but should be fine in the situations in which I plan to use it.

I decided to house this stand-alone circuit in one of the plastic jewel boxes I had used before for the Active Tone Control and Low-Pass Filter, as mentioned above, and the Touch-Radio. Because of the minimal component count and no circuit board, even with the voltage regulator module, there was no problem fitting everything in the box:

The completed unit looked like this:

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After trying the circuit out in this way, I decided to put one of these units into each of the percussion instruments, and in the case of the piezo-based ones, I made up the preamp circuit boards with the PT2399’s included.

I wanted to be able to adjust the number of repeats this time, so I experimented a bit and came up with the following development of the circuit above. Since making the drawing and further experimenting with the percussion instruments, I changed the ‘REPEATS’ potentiometer to 50k; in some cases, depending on the sound from the instrument itself, I increased the added resistance here from 15k to 20k:

In this case I experienced problems connecting the preamp output directly to the input of the PT2399 circuit; so I added a 2k resistor (actually two 1k resistors in series) between the preamp output and PT2399 input – point ‘A’ in the above diagram.

It was at this point when I noticed that the output sound of the PT2399 didn’t include the sound at the input! . . . So, in order to hear the original as well as the delayed version, I used the other half of the TL072 as a simple mixer – it’s a dual op amp chip, and only one of them is used for the piezo preamp.

The circuit looked like this:

Although, in fact, the PT2399 output required 200k-500k, depending on the device, to balance the volume with the ‘dry’ signal. The power connections – 4.5v to pin 8, 0v to pin 4, and 2.25v to pin 5 – were already in place for the half of the TL072 used for the piezo preamp).

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I added one of these boards to each of the four piezo percussion units, plus a volume control between the mixer and the output socket – the odd one out being the one for the rainstick, which was designed to be a stereo device. This meant doubling up each of the elements of the circuit: 2 preamps, 2 PT2399’s and 2 mixers; and using dual potentiometers for delay time, repeats and volume. As the stereo preamp used both halves of the TL072, a second one was needed for the mixer left and right channels.

Here’s a sound file of the ‘Snare’ instrument, the Piano String instrument and the Sandpaper instrument. Bear in mind that these are not being struck with any force: on the contrary, they’re being tapped very lightly with a thin disposable wooden coffee stirrer.

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I wanted to do one more thing with the Sandpaper instrument before moving on. The scratching sound covered a wide frequency range, and I thought it would be interesting to vary the sound by putting it through a low pass filter.

This circuit was placed after the mixer stage described above. I added a 1k resistor at the input, and at the output a 1M preset and a 10uF capacitor. The only change I made to the circuit as shown was to use a 500k potentiometer in place of the 1M potentiometer for the cut-off frequency, and didn’t include a ‘Fine’ control.

This picture shows how few components are needed to make an excellent filter. There are two circuits on this small board:

This filter proved very effective, and sounded like this:

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I then moved on to the instruments with the plastic bottles and the electrets. I glued the electrets to the acrylic tubes, and the tubes to the wooden bases, then connected the electrets to the circuit boards I had prepared, each containing both a preamp and a PT2399 Echo circuit.

After constructing some preamp circuits for electret microphones, as described in the first article in this series, I started to look at different uses for them.

First of all, I had some conventional instruments to amplify – a xylophone and a glockenspiel; secondly, I wanted to make percussion instruments from some plastic bottles.

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I had a collection of plastic bottles, which would be suitable for tuned (or semi-tuned) percussion. I sawed the ends off, leaving them at different lengths – and therefore sounding at different pitches – and prepared a framework to attach them to. This consisted of small square trays which I bought, and 2x2cm wood, which I cut to length.

I then glued the bottles to each side of the central post:

Each bottle would have an electret microphone inside. The electret elements were salvaged from part of a job lot of voice memo recorders which I bought in bulk on eBay. These were said to be non-working, but their only problem seemed to be that the coin-type batteries had run down. The electret element can be seen in position at the top of the right-hand picture below.

The electrets were to be mounted inside the plastic bottles on short lengths of rigid acrylic tubing.

The pictures below give an idea of how the electrets attach to the tubes, and the tubes attach to the instruments’ bases:

Following on from this post, I will describe the xylophone and glockenspiel which also needed an economical method of amplifying; and then installing the electronics for all the new percussion instruments.